Key is invalid. You must supply a key in OpenSSH public key format

[anatoly-kor]

I'm try paste private key in Deploy Keys in settings and have error: Key is invalid. You must supply a key in OpenSSH public key format.

As described above you will need to create a Deploy Key, place the the private part in the Deploy Key settings of the subtree repository; and the public part in the Secrets settings of the parent repository.

Is this part from README is relevant?

[MPParsley]

I'd say it's the other way round, no?